On-Line Thinning Measurement in the Deep Drawing Process

The conventional deep drawing process is limited to a certain Limit Drawing Ratio (LDR), beyond which localized wall thinning and rupture occur. One way to increase the LDR is to try to capture the onset of necking and to adjust process parameters in order to delay or avoid necking. This paper describes a method for monitoring the wall thickness of a cup during the deep drawing process. Measurement utilizes a noncontact ultrasound gauge that is located orthogonally to the drawn cup’s wall and is immersed in oil to create an acoustic coupling. Monitoring is based upon a deep drawing process model using a thin blank with a round cross-section. Thickness distribution along a longitudinal axis is predicted and is used as a trajectory to track in-process thinning variations that may lead to tearing. The robustness of the measurement system is examined by applying the technique in different experiments. Results show that in-process measurements correlate well with grid strain analysis of a formed sheet metal part.